The physical basis of the lens opacity of the cataract is spacial fluctuations in the refractive index in the lens. We found that one of the causes of the fluctuations is the formation of high protein concentration domains inside the lens fiber cells caused by phase separation of the lens cytoplasm. We have demonstrated that the nuclear opacity of the galactosemic cataract is also a manifestation of cytoplasmic phase separation. We are currently studying cataracts associated with x-ray irradiation and hypoglycemia. As a model of the hypoglycemic cataract, we are studying the opacification of the isolated lens cultured in glucose-deficient media. For cataract genesis induced by x-ray irradiation, we found a measurable change of protein diffusivity before the appearance of opacity using the laser light scattering technique. We will investigate the relation between the change of diffusivity and the amount of irradiation. This work can be applied to in vivo detection of cataract susceptibility before the appearence of lens opacity. We are also studying two model systems: phase separation of protein-salt water mixtures and phase separation of ionic gels.